1. Field of the Invention
The present invention relates to an occupant detection system, which detects the presence or absence of the occupant on the vehicle seat.
2. Description of Relevant Art
As an example of an apparatus for detecting the presence or absence of the occupant on the vehicle seat, the apparatus, in which the presence or absence the occupant is detected by measuring the weight applied to the vehicle seat, can be cited. Among these kinds of apparatuses, the seat weight measuring device disclosed in Japanese unexamined patent publication JP2000-258233 can be cited as an example.
This weight measuring device disclosed in this publication has a load sensor and a deflection absorbing mechanism. The load sensor converts the weight of the vehicle seat into an electrical signal. The deflection absorbing mechanism is provided between the vehicle seat and the load sensor. This deflection absorbing mechanism absorbs the creep or deflection caused by the weight applied to the vehicle seat, and transmits moment of force brought out from the weight through an arm.
The load sensor disclosed in this publication is a strain gage composed of electric wirings and a plate spring, and is joined with a movable part of the arm. In this weight measuring device, one end of this arm is fixed at the vehicle side, and the other end of this arm is connected to the strain gage (load sensor) through a movable part Here, this movable part is displaced when the arm is turned by the load applied from the rail of the vehicle seat.
Thus, when the vehicle seat receives a load and the movable part of arm is moved as a result of the turn of the arm, a moment of force is applied to the load sensor. In this occasion, the amount of the load applied to the vehicle seat is measured by processing the electrical signal, which is obtained from the strain gages and is fluctuated in compliance with the amount of force.
The deflection absorbing mechanism is composed of a pin bracket and an arm. The pin bracket, which is positioned between the rail and arm, is joined with the arm through a pin with play.
By providing this play, the accidental deformation or the manufacturing error in the size of the vehicle parts is absorbed. Thus, the deflection absorbing mechanism does not transmit the load caused by the deformation to the load sensor 3.
In the conventional apparatus which measures the weight of the vehicle seat, however, if the ambient air temperature changes rapidly, the value of the resistance (hereinafter indicated as resistance value) of some strain gage may vary from other strain gages. This is because the heat transfer rate of each parts of the plate spring is different from other parts.
In this case, the resistance value obtained by the measurement is affected by the ambient air temperature due to the difference of the heat transfer rate.
Therefore, it has been required that the occupant detecting system perform the accurate measurement irrespective of the change of the ambient air temperature.